In some vehicles, particularly farm tractors and the like, the hydraulic braking system includes two master cylinders operating as pumps, which are controlled by two separate braking pedals. Each of these master cylinders operates the braking of one of the rear side wheels (left-hand and right hand wheels) of the vehicle. This arrangement has the purpose of allowing a different braking of the side wheels, for example in order to help the vehicle rotation by the U-turn when a field is being worked along adjacent lanes which are run in alternatively opposite directions. When, on the contrary, a bilateral braking is controlled by operating both the braking pedals, in general the system could be unbalanced. because the two separate braking circuits take up different oil quantities, and the result would be a similarly unbalanced braking. In order to prevent this drawback, a transfer channel is provided that mutually communicates the two braking circuits when both the braking pedals are operated simultaneously. The balancing function is effected by balancing valves installed on the pistons of the two master cylinders, these valves normally intercepting the intakes of the transfer channel, and opening these intakes when the pistons have covered a preestablished stroke.
Therefore, in the absence of any braking action, both the intakes of the transfer channel are closed, and the braking liquid contained in this channel is confined in a closed space. In these conditions, when the transfer channel undergoes a temperature increase, the contained liquid is thermally dilated, and its volume increase causes in the closed space a pressure increase. In particular conditions, this pressure increase may take important values which are capable of damaging the balancing valves.
This phenomenon takes place, in particular, when the master cylinders, the balancing valves and the transfer channel are installed in hot regions of a farm tractor, for example within the motor hood of a coach-worked tractor, and the operation takes place in static conditions with insufficient cooling, that is the case of the operation of a water-scooping machine or other implement. In general it appears that this phenomenon takes place as a result of a temperature increase of about 60° starting from the last braking operation effected.